When slice machining a wafer from a pillar-shaped workpiece by a wire discharge-machining apparatus, there has been proposed an approach to improve productivity of slice machining the workpiece by simultaneously generating electric discharges between the workpiece and cutting wires by individually passing a current to each cutting wire among many cutting wires arranged by winding a wire electrode in parallel between plural guide rollers (see, for example, Patent documents 1 and 3).
In a semiconductor wafer manufacturing apparatus having the above configuration, magnetic fields are generated around parallel wire electrodes when a machining current flows to the wire electrodes, an electromagnetic force works on adjacent wire electrodes, and then warps the wire electrodes in some cases. On the other hand, in a conventional semiconductor wafer manufacturing apparatus such as that described in Patent document 1, currents are supplied from both sides of the workpiece to parallel wires. Therefore, no countermeasure is taken against warping of wire electrodes due to the electromagnetic force of the wire electrodes.
Meanwhile, regarding the electromagnetic force working on the wire electrodes, for example, Patent document 2 describes, in a configuration of a wire discharge-machining apparatus, a method of using an electromagnetic force in an operation to return a wire electrode to a direction of avoiding a short-circuit when the wire electrode is short-circuited with a workpiece. That is, magnetic fields are generated around a workpiece by passing an auxiliary current to the workpiece, and the wire electrode is restored by using the magnetic fields and an electromagnetic force of a machining current flowing to the wire electrode. By controlling the strength and flow direction of an auxiliary current according to a machining state, the strength of magnetic fields is changed to control an electromagnetic force working on the wire electrode.
However, in the wire discharge-machining apparatus having a configuration such as that described in Patent document 2, the apparatus does not assume magnetic fields working between wire electrodes by passing a machining current to parallel wire electrodes arranged by winding a wire electrode around plural guide rollers like in the present invention. Therefore, in the control of an auxiliary current to the workpiece, an electromagnetic force working on the wire electrode cannot be reduced by the magnetic fields described above. Further, controlling sizes and flow directions of auxiliary currents supplied to the workpiece during machining results in changing energy of electric discharge machining, and this becomes a cause of degrading machining precision and reducing its machining speed.
The technique proposed in Patent document 2 is to improve an avoiding operation of a short-circuit state, but does not become a method of preventing warping of wire electrodes due to an electromagnetic force when machining using a wire electrode.